Alkali metal sulfates are well known inorganic compounds which have found great utility in many areas such as in agricultural chemicals and in detergent compositions. Potassium sulfate and sodium sulfate are also both well known as inorganic industrial chemicals which are also used as intermediates for production of many other useful products.
As a result of the need for such products, the prior art has been aware of various methods for preparation of these materials for many years. In a common procedure, alkali metal sulfates are prepared by the reaction of alkali metal chloride and sulfuric acid. Usually, however, it is necessary to carry out such reactions under rather severe reaction conditions in order to eliminate the hydrogen chloride evolved during the reaction and shift the reaction toward production of the alkali metal sulfate. An inherent problem in carrying out this process is that it is a two-step reaction involving an intermediate reaction in which the alkali metal chloride and sulfuric acid react to form alkali metal bisulfate or alkali metal hydrogen sulfate with the elimination of one mole of hydrogen chloride. It has been difficult, however, to carry this reaction to completion and eliminate the additional mole of hydrogen chloride because of the thermodynamics involved in the reaction. For these reasons, it has been necessary for the art to utilize rather extreme reaction conditions, for example, temperatures well above 100.degree. C. in order to obtain an economically attractive yield of alkali metal sulfate of reasonable purity. These extreme reaction conditions, however, merely cause additional problems as the strong sulfuric acid and chlorine present at the high temperatures cause severe corrosion problems.
In most of the prior art procedures, the key to a successful process resides in complete elimination of the HCl gas. A large body of art is available which discloses such reactions involving the production of K.sub.2 SO.sub.4 or Na.sub.2 SO.sub.4. A typical art reference of this type is U.S. Pat. No. 2,762,689 which involved the reaction of NaCl and H.sub.2 SO.sub.4 at temperatures of 200.degree. to 250.degree. C. Also in U.S. Pat. No. 2,275,825, Na.sub.2 SO.sub.4 and hydrochloric acid are prepared by the reaction of a slurry comprising water, NaCl, NaHSO.sub.4 and H.sub.2 SO.sub.4, with steam in a counter-current reaction zone at temperatures of 120.degree. to 160.degree. C. With respect to prior art of this type, it should be noted that the aqueous sulfuric acid systems of Na.sub.2 SO.sub.4 and K.sub.2 SO.sub.4 are not generally considered to be equivalent in the recovery of desired products as they behave differently. Thus, K.sub.2 SO.sub.4 forms mixed acid salts on solution/crystallization whereas Na.sub.2 SO.sub.4 forms hydrates of either Na.sub.2 SO.sub.4 or NaHSO.sub.4, "Solubilities of Inorganic and Metal Organic Compounds", Seidell 4th ed., vol. 2, pp. 1126-27.
A prior art disclosure related to the present invention is U.S. Pat. No. 3,687,639 which discloses a process of precipitating K.sub.2 SO.sub.4 crystals from an aqueous solution of K.sub.2 SO.sub.4 by dissolving sufficient KCl in the solution to affect the solubility of the K.sub.2 SO.sub.4 and cause it to crystallize. However, this is strictly a physical procedure as the K.sub.2 SO.sub.4 is already formed. A similar reference with respect to sodium sulfate recovery is U.S. Pat. No. 2,125,624. However, these patents are not concerned with the particular reaction of the present invention.
Partial solutions to these problems are found in U.S. Pat. No. 3,998,935 and Japanese Pat. No. 42-27246. In these prior patents, potassium sulfate is produced by the reaction of potassium chloride in an aqueous solution containing potassium hydrogen sulfate at temperatures of about 65.degree.-110.degree. C., cooling the solution and permitting the potassium sulfate to crystallize from solution. While these reactions can be carried out at moderate temperature conditions, it has been found that problems are caused by excessive buildup of HCl and the requirement for evaporation of large amounts of water.
Other patents which relate to the production of alkali metal sulfates and hydrogen chloride include British Pat. No. 1,145,390, U.S. Pat. No. 995,105, U.S. Pat. No. 2,816,820, and U.S. Pat. No. 2,902,341. These patents disclose reactions for obtaining alkali metal sulfates from aqueous solutions.
There accordingly remains a need in the art for a process for the production of alkali metal sulfates from reactions systems which involve moderate reaction conditions and with reusable reactants. The present invention provides a solution to this need.